Directional over current protection


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In 3 phase Electromechanical directional over current relay what will happen if one of the PT voltage fails? With available 2 phases will the relay trip during power reversal?

Phil Corso, PE

Responding to zaakeer's 26-Dec-06 12:15 pm query... electromagnetic directional overcurrent relays essentially measure power, that is, V x I (not related to system power). Thus, to determine the direction (to the fault location) voltage is used as the discriminating parameter. But, without specific relay and system parameters (for example you did not identify whether the relay is for ph-ph faults or ph-earth faults, I can only provide a general response:

A) The Single-Phase Relay.
Directional overcurrent relays are torque-operated. It has two operational windings: one is fed with a current signal; the other the voltage signal. When the flux deived from the voltage signal equals the flux derived from the current source no torque is produced to turn the disk. So, if the phase-neutral voltage source is different than that for phase-current (for example A-n voltage witb B-ph current) then it is unlikely to trip falsely.

B) The Three-Phase Relay.
Except for a local fault, then those located far from the relay will produce a myriad of torques that vary widely, but the quantity of PTs is important. If 3 PTs are used to measure ph-ph voltages it is unlikely that loss of one will cause a false-trip. However, if only 2 PTs whose secondaries are open-delta connected are used, then the probability is higher that a false-trip will occur.

If additional detail is required, send particulars.

Regards, Phil Corso, PE